Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface, showing my new design, according to a first embodiment thereof; and,

FIG. 2 is a front view of a display screen or portion thereof with a graphical user interface, showing my new design, according to a second embodiment thereof.

The broken lines illustrate the display screen and environmental graphical elements not included in the graphical user interface, and the broken lines form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 